The present invention pertains to an improved tool for tightening a strap around an object or load and adhering the strap onto itself. More particularly, the present invention is directed to a dual pneumatic motor tool for tensioning a strap around a load and welding or melt-adhering the strap onto itself.
Strapping tools are well-known in the art. These tools come in a wide variety of types, from fully manual tools to automatic, table-top tools. These tools are generally specifically designed for use with metal strapping or plastic/polymeric type strapping.
Typically, strappers for applying plastic or polymeric strapping materials are automatic table-top or hand-held devices. This is generally necessary in order to provide energy for adhering the strapping material onto itself. Typically, the adhering function is provided by melting or otherwise welding a section of the strapping material onto itself. Such melting or welding operations are generally carried out using ultrasonic or vibrational-type weld assemblies. The movement or vibrational motion can be provided by electrical, electromechanical or fluid drive (hydraulic or pneumatic) systems.
In one exemplary tool, a pneumatic system is used to vibrate contacting interfacial surfaces of overlapping plastic strap portions. The tool includes a clamping member for anchoring a strap free end portion, and a rotatable feed wheel cooperating with an anvil foot to engage a feed strap portion that is tensioned about the load. A clutch engagably couples the feed wheel to a pneumatic motor which also vibrates a jaw that welds the overlapping strap portions.
In this single motor arrangement, the motor stalls and tensioning stops at a specified strap tension. A pneumatically actuated ram disengages the motor from the feed wheel and engages the feed wheel break that maintains feed wheel tension on the tensioned strap during welding. Such a tool is disclosed in U.S. Pat. No. 3,564,033 to Angarola et al., which patent is assigned commonly herewith, and is incorporated herein by reference. The actuated ram also moves the vibrating jaw into frictional engagement with the overlapping strap portions. As the vibrating jaw moves toward the overlapping strap portions, a sheering edge severs the upper strap and the overlapping strap portions are subsequently welded together. The pressure is maintained on the welded overlapping strap portions for a cool down period to complete the weld.
Another exemplary tool, disclosed in U.S. Pat. No. 5,380,393 to Drabarek et al., also assigned commonly herewith and incorporated herein by reference, discloses a strapping tool having a pneumatic circuit for automatically controlling tool operation, including the timing and duration of strap welding and the cool down period. A pneumatically actuated ram extended after strap tensioning pivots a cam that moves a vibrating weld plate into contact with overlapping strap portions to form the weld. Vibration of the weld plate terminates after a time period controlled by the accumulation of air in a chamber. Air is subsequently bled from a cylinder that counter-pivots the ram to move the welding plate away from the welded strap portions after the cool down period.
As will be recognized by those skilled in the art, known pneumatic strapping tools require many system components that increase the size and weight of the tool. In addition, even though the pneumatic circuit of the above-mentioned U.S. Pat. No. 5,380,393 automates many strapping operations otherwise performed manually, controlling timing of the various tool operations as relatively and precise, and as a result inconsistent strap tension and welds may result.
In operation of strapping tools, after tension is applied to the strap, and prior to welding, the tension must be slightly released so that the (welded) strap can be removed from the tool. Otherwise, the tension is typically so high and the foot or anvil is “pinched” so tightly between the strap and the load, that the tool is extremely difficult to remove without damaging the strap.
In order to accomplish the release or backing-off of the tension, in many known strapping tools the tensioning motor is reversed or gearing is used to permit roll back of the feed wheel. As such, either the feed (tensioning) motor must be a two-way motor or complex gearing is required to carry out the necessary functions of the tool.
Accordingly, there exists a need for a pneumatic strapping tool that uses two pneumatic motors, one motor for tensioning or feeding strap and another, separate motor for welding the strap material onto itself. Desirably, such a tool uses one-way or unidirectional pneumatic motors. Most desirably, such a tool has a self contained modular pneumatic member or module that includes pneumatically controlled timing circuits and is readily installed onto and removed from the body and motor portions of the tool. Even more desirably, such a tool incorporates guide elements to properly position and align the strap material between the anvil and the feed wheel to assure optimal strap welding.